Constant advances of semiconductor manufacturing technology have greatly shrunken the size of electronic elements and also greatly reduced their production costs. Conventional semiconductor manufacturing techniques generally adopt etching, ion implantation or wiring on substrates to form planar semiconductor structures. These techniques now can fabricate a chip at a minimum size of 6F2. However, as development speed of minimizing feature size has gradually slowed down, the occupied area of semiconductor on wafers is difficult to shrink. As a result, development of vertical (also called three-dimensional) semiconductor manufacturing technology has progressed rapidly. It aims to grow semiconductors vertically on the wafer to reduce the occupied area of transistors on the wafer surface and can further shrink the chip dimension to 4F2. For instance, U.S. Pat. No. 7,326,611 entitled “DRAM arrays, vertical transistor structures and methods of forming transistor structure and DRAM Arrays” and U.S. publication No. 2005/0190617 entitled “Folded bit line DRAM with vertical ultra thin body transistors” disclose a vertical pillar transistor structure, manufacturing method and processes. It has a pillar and a gate material at one side to control ON/OFF of the pillar functioned as a transistor. The gate material usually is a metal wire formed by etching to attach to two sides of the pillar without connecting to each other. With the present technique capable of reducing the feature size to 40 nm, etching metal wires on two sides of the pillar to form the gate material encounters a great challenge due to controlling thickness is difficult.
U.S. publication No. 2009/0256187 entitled “SEMICONDUCTOR DEVICE HAVING VERTICAL PILLAR TRANSISTORS AND METHOD FOR MANUFACTURING THE SAME” discloses a technique that forms a gate only at one side of the pillar. It etches the pillar to form a trench and disposes metal in the trench to form a gate. It provides a fabrication method different from the previous techniques by doing away etching of the metal wires to avoid the problem of thickness control of the metal wires. But forming the gate still needs the etching process, and forming the trench on the pillar by etching also has the same kind of difficulty.